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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to a method for controlling ignition timing in an
internal combustion engine. More particularly, the invention relates to a
method for controlling ignition timing in an internal combustion engine
provided with an exhaust gas recirculation device.
In order to reduce the contents of harmful components in the exhaust gas of
an internal combustion engine and to obtain lower fuel consumption, it is
necessary to effectively control the spark-advance of the engine in
response to the engine's operating conditions. Since this optimum
spark-advance angle changes depending mainly on the rotational speed of
the engine and on the value of the absolute pressure in the intake
manifold of the engine, namely, on the load of the engine, according to
the conventional technique, a governor advance mechanism is used for
setting the engine speed advance angle and a vacuum advance mechanism is
used for setting the engine load advance angle. An apparatus is also used
together with a digital computer for electronically controlling the engine
speed advance angle and the engine load advance angle.
In an internal combustion engine provided with an exhaust gas recirculation
device (hereinafter referred to as an EGR device), even if the
spark-advance angle is controlled by taking the engine rotational speed
and the engine load into consideration, a sufficient optimum spark-advance
angle cannot be obtained at all.
In order to eliminate this defect occurring in the conventional technique,
the applicant previously proposed a method in Japanese patent application
No. 51-81,890 for controlling the spark-advance angle in an internal
combustion engine provided with an EGR device by using a digital computer.
According to this method for controlling the spark-advance angle, the
total amount of gas sucked into the engine is obtained by calculation of
the measured rotational speed and intake pressure. The ratio of the amount
of intake air to the amount of recirculated exhaust gas (hereinafter
referred to as EGR gas), namely the EGR ratio, is calculated from the
result of such calculation and the amount of actually measured intake air.
An optimum spark-advance angle is determined by using the so calculated
EGR ratio, and the ignition timing is controlled based on the so
determined optimum spark-advance angle.
However, by using the above-mentioned method, the risk of a serious error
occurring in the EGR ratio is high especially at low engine rotational
speed since discrepancies and errors readily occur between the calculated
amount and the actual amount of the total gas. Such error in the EGR ratio
can cause a serious error in the spark-advance angle controlled in
proportion to the EGR ratio. In such case, if the determined optimum
timing angle is too large or too small, serious problems will be caused in
the operation of the engine.
SUMMARY OF THE INVENTION
It is therefore a primary object of the present invention to provide a
method for controlling the ignition timing in an internal combustion
engine provided with an EGR device, whereby the ignition timing can be
controlled precisely with much certainty and with no substantial error.
According to the present invention, a method for controlling ignition
timing in an internal combustion engine comprises steps for measuring the
amount Q of air sucked into the engine and the value P.sub.B of the
absolute pressure in an intake manifold of the engine when engine exhaust
gas is being recirculated; for calculating the value P.sub.BO of absolute
pressure in the intake manifold when exhaust gas is not being
recirculated, which calculating step being performed by using the measured
intake air amount Q and the measured rotational speed N of the engine; for
calculating an optimum spark-advance angle .alpha..sub.O in the engine
when engine exhaust gas is not being recirculated, which calculating step
being performed by using the measured intake air amount Q and the measured
rotational speed N of the engine; for calculating an exhaust gas
recirculation ratio X using the measured absolute pressure value P.sub.B
in the intake manifold and the calculated absolute pressure value P.sub.BO
in the intake manifold; for calculating an optimum spark-advance angle
.alpha. in the engine when engine exhaust gas is being recirculated, the
above-mentioned calculating step being performed by using the calculated
optimum spark-advance angle .alpha..sub.O in the engine and by using the
calculated exhaust gas recirculation ratio X of the engine; and the step
for controlling ignition timing of an ignition system of the engine in
response to the calculated optimum spark-advance angle .alpha..
In the preferred embodiment of the present invention, the aforementioned
steps for calculating an absolute pressure value P.sub.BO and for
calculating an optimum spark-advance angle .alpha..sub.O respectively
include the step for calculating an absolute pressure value P.sub.BO by
means of a digital computer programmed to calculate the value P.sub.BO
from functions describing a desired relationship between the amount of air
taken into the engine, the rotational speed of the engine and the value of
absolute pressure in the intake manifold when engine exhaust gas is not
being recirculated; and the step for calculating an optimum spark-advance
angle .alpha..sub.O by means of the digital computer which is also
programmed to calculate the angle .alpha..sub.O from functions describing
a desired relationship between the amount of air taken into the engine,
the rotational speed of the engine, and the optimum spark-advance angle of
the engine when engine exhaust gas is not being recirculated.
In a further preferred embodiment of the present invention, the
above-mentioned step for calculating an exhaust gas recirculation ratio X
includes the step for calculating an exhaust gas recirculation ratio X by
means of the digital computer which is programmed to calculate the ratio X
from a specific algebraic function. It is preferable that this specific
algebraic function be defined as X=(P.sub.B -P.sub.BO)/P.sub.B.
In another further embodiment of the present invention, the above-mentioned
step for calculating an optimum spark-advance angle .alpha. includes the
step for calculating an optimum spark-advance angle .alpha. by using the
digital computer which is programmed to calculate the angle .alpha. from a
specific algebraic function. It is preferable that this specific algebraic
function be defined as .alpha.=.alpha..sub.O +KX, where K is a constant.
The above and other related objects and features of the present invention
will be apparent from the description set forth below with reference to
the accompanying drawings and also from the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of an internal combustion engine to which one
embodiment of the present invention is applied;
FIG. 2 is a diagram illustrating control procedures of the present
invention;
FIG. 3 is a block diagram of a control circuit shown in FIG. 1; and
FIG. 4 is a diagram illustrating wave-forms obtained at various points in
the control circuit shown in detail in FIG. 3.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1 which schematically illustrates an internal combustion
engine to which one embodiment of the present invention is applied, an EGR
pipe 4 is disposed between an exhaust pipe 2 and an intake pipe 3 of an
internal combustion engine 1. A part of the exhaust gas in the exhaust
pipe 2 is recycled to the intake pipe 3 through the EGR pipe 4. The amount
of the recycled exhaust gas is controlled by changing the sectional area
of the path of an EGR valve 5 disposed midway of the pipe 4.
An air flow sensor 7 is disposed upstream of a throttle valve 8 in the
intake pipe 3, which is disposed upstream of an EGR gas outlet port 6, and
a negative pressure detecting port 9 is disposed downstream of the outlet
port 6 in the intake pipe 3. A negative pressure sensor 10 is connected to
this negative pressure detecting port 9. The air flow sensor 7 sends out
an analog voltage level which is proportional to the intake air amount Q
of the engine. This output voltage is then applied to an electronic
control circuit 11. The negative pressure sensor 10 consists of a member
which sends out an analog voltage of a level which is proportional to the
absolute pressure value P.sub.B in an intake manifold 17, namely
downstream of the throttle valve 8 in the intake pipe 3, for example, a
pressure-responsive semi-conductor element. This output voltage is applied
to the electronic control circuit 11.
Crank angle position sensors 12 and 13 are attached to a crankshaft (not
shown) of the engine 1. Each of the sensors 12 and 13 comprises a disc
rotating together with the crankshaft and a magnetic pick-up transducer or
the like disposed in the vicinity of the periphery of the disc.
Projections are formed on the peripheral end of the disc of the sensor 12
at intervals of a certain crank angle such as 1.degree.. Accordingly, a
pulse is generated from the magnetic pick-up transducer of the sensor 12
every time the crankshaft rotates by a certain crank angle such as
1.degree.. Separately, projections are formed on the peripheral end of the
disc of the sensor 13 at intervals of 180.degree. so that the top dead
center (TDC) of the engine appears every time the projection passes the
vicinity of the magnetic pick-up transducer. Output pulses of the crank
angle position sensors 12 and 13 are applied to the electronic control
circuit 11.
The output terminal of the electronic control circuit 11 is connected to
respective spark plugs 16a to 16d through an igniter 14, and a distributor
15.
FIG. 2 is a diagram illustrating procedures for calculating the
spark-advance angle in the ignition timing control method of the present
invention. As shown in FIG. 2, the actual intake air amount Q and the
rotational speed N of the engine occurring when exhaust gas is being
recirculated are first detected, and then the intake manifold absolute
pressure value P.sub.BO and the optimum spark-advance angle .alpha..sub.O
occurring when exhaust gas is not being recirculated are determined from
the predetermined functions of P.sub.BO =f(Q, N) and .alpha..sub.O =g(Q,
N). Then, the actual intake manifold absolute pressure value P.sub.B of
the engine occurring when the exhaust gas is being recirculated is
detected, and the EGR ratio X is thereby calculated from the formula
X=(P.sub.B =P.sub.BO)/P.sub.B. Finally, the optimum spark-advance angle
.alpha. occurring when exhaust gas is being recirculated is calculated
from the formula .alpha.=.alpha..sub.O +KX. In the above formula, K stands
for constant value which is determined by the engine operating condition
and ambient condition such as air-fuel ratio, coolant temperature and
barometric pressure.
FIG. 3 is a block diagram illustrating in detail the electronic control
circuit 11 in the embodiment shown in FIG. 1. The structure and operation
of the apparatus of the present embodiment will now be described with
reference to this block diagram and to the wave-form diagram of FIG. 4.
The output terminal of the crank angle position sensor 12 is connected to
the clock pulse input terminal of a binary counter 21 through an AND gate
20. The output terminal of a pulse generator 22 for generating pulses
having a predetermined duration is connected to the other end of the AND
gate 20. Accordingly, pulses generated, for example, at every 1.degree.
crank angle from the crank angle position sensor 12 are allowed to pass
through the AND gate 20 for a predetermined time, and then such pulses are
counted by the binary counter 21. Namely, the output of the binary counter
21 is a value proportional to the rotational speed N of the engine.
The analog signal of a level which is proportional to the intake air amount
Q of the engine, which signal is sent from the air flow sensor 7, is
digitized by an A/D converter 23 which is disposed after the air flow
sensor 7.
The analog signal of a level which is proportional to the intake manifold
absolute pressure value P.sub.B of the engine, which signal is transmitted
from the negative pressure sensor 10 (also called a vacuum level sensor),
is digitized by an A/D converter 24 disposed after the negative pressure
sensor 24.
The output terminals of the binary counter 21 and the A/D converters 23 and
24 are connected to an interface 25d of a digital computer 25. This
digital computer 25 consists of, for example, a commercially available
Micro-Computer MCS-8 including a micro-processor 25a such as Intel 8080 or
8008, a read-only memory (ROM) 25b, a random access memory (RAM) 25c,
interfaces 25d and 25e and an input-output (I/O) device 25f for
controlling the interfaces 25d and 25e. The storage and processing
capacity of such Micro-Computer MCS-8 are much greater than the capacities
required in the present invention. Accordingly, for carrying out the
present invention, a custom-made digital computer may be used so as to
reduce the cost and space required.
An optimum value of the spark-advance angle .alpha..sub.O occurring when
the exhaust gas is not recirculated, which value expressed as a function
of the intake air amount Q and the rotational speed N of the engine, is
stored in ROM 25b of the digital computer 25. The relation of
.alpha..sub.O =g(Q, N) can readily be determined in advance by
experiments. The relation of P.sub.BO =f(Q, N) among the intake air amount
Q, rotational speed N and intake pipe absolute pressure value P.sub.BO of
the engine occurring when EGR is not being effected is stored in ROM 25b.
This function P.sub.BO =f(Q, N) is also determined in advance by
experiments. The determined function can be expressed, for example, as
P.sub.BO =a(Q/N)+b, where a and b are constants.
The digital computer 25 first reads various input data, namely the intake
air amount Q, rotational speed N and intake manifold absolute pressure
value P.sub.B, through the interface 25d, and they are stored at
predetermined address positions of RAM 25c. By using the above-mentioned
input data Q and N, the intake manifold absolute pressure value P.sub.BO
occurring when the exhaust gas is not being recirculated is calculated
from the function f(Q, N) stored in ROM 25b, and the EGR ratio X is
determined from the calculated value of the intake manifold absolute
pressure value P.sub.BO and the input data P.sub.B according to the
formula X=(P.sub.B =P.sub.BO)/P.sub.B. Then, an optimum spark-advance
angle .alpha..sub.O occurring when the exhaust gas is not being
recirculated is determined by using the input data Q and N from the
function g(Q, N) stored in ROM 25b, and from this .alpha..sub.O and the
above EGR ratio X, an optimum spark-advance angle .alpha., when the
exhaust gas is being recirculated is calculated according to the formula,
.alpha. =.alpha..sub.O +KX. As pointed out hereinbefore, K is a constant
which is usually assigned a value residing within the range of from about
0.5 to about 1.0. It is known in the art that when the exhaust gas is
recirculated, an optimum spark-advance angle can be obtained by increasing
the spark-advance angle in proportion to the EGR ratio, namely, in
proportion to the amount of the inert gas in the cylinder. For example, it
is known that when the EGR ratio is increased by 1%, good results can be
obtained if the spark-advance angle is made larger than the optimum
spark-advance angle occurring when the exhaust gas is not being
recirculated by about 1.degree..
After calculation of the spark-advance angle .alpha. is finished, the
micro-computer 25 calculates the output data concerning the ingition
timing and the timing for initiating application of electricity to the
ignition coil. In the present embodiment, since the engine is of the
four-cylinder four-stroke cycle type, the standard ignition point thereof,
namely, the top dead center, appears at every crank angle of 180.degree.,
and each of the clock pulses from down-counters 26 and 27 described
hereinafter has a frequency corresponding to a crank angle of 1.degree..
Accordingly, the ignition timing data can be expressed as
(180.degree.-.alpha..sub.O)/1.degree.. Supposing that the predetermined
dwell angle is .beta., the timing for initiating application of
electricity to the ignition coil occurs later by a crank angle of
(180.degree.-.beta.) than the ignition timing in the preceding cylinder.
Accordingly, the timing data for initiating application of electricity to
the ignition coil can be expressed as (180.degree.-.beta.)/1.degree..
Data input terminals of the down-counters 26 and 27 which can be preset are
connected to the digital computer 25 through the interface 25e, and clock
pulse input terminals thereof are connected to the output terminal of the
above-mentioned crank angle position sensor 12 through AND gates 28 and
29, respectively. The load signal input terminal of the down-counter 26 is
connected to the output terminal of the above-mentioned crank angle
position sensor 13. The output terminals of the down-counters 26 and 27
are connected to input terminals of zero detectors 30 and 31,
respectively. The output terminals of the zero detectors 30 and 31 are
connected to the other input terminals of the AND gates 28 and 29,
respectively. The zero detectors 30 and 31 are arranged so that each of
the detector generates a low level output when the output value of the
respective down-counters 26 and 27 is zero, and each of the detectors
generates a high level output when the output value of the respective
down-counters 26 and 27 is a value other than zero.
When a signal indicating the top dead center (TDC) in the engine [FIG.
4-(B)] is applied to the down-counter 26 from the crank angle position
sensor 13, the down-counter 26 receives the above-mentioned ignition
timing data from the digital computer 25 as a presetting value thereof. At
this point, because the output [FIG. 4-(C)] of the zero detector 30 is a
high level output the AND gate 28 can thereby be opened. Then, the clock
pulse [FIG. 4-(A)] is applied to the down-counter 26, and the counting
operation is performed. When the output value of the down-counter 26 is
reduced to zero, the output of the zero detector 30 is changed to a low
level output and a negative leading edge trigger circuit 32, which is
connected to the output terminal of the zero detector 30 and which is
comprised of, for example, a differentiation circuit, is therefore
energized. When the output pulse [FIG. 4-(D)] of the trigger circuit 32 is
applied to the down-counter 27, it receives the above-mentioned data of
the current application starting point from the digital computer 25 as a
presetting value thereof. At this point, because the output [FIG. 4-(E)]
of the zero detector 31 is a high level output, the AND gate 29 is opened,
the clock pulse [FIG. 4-(A)] is applied to the down-counter 27, and
counting is performed. When the output value of the down-counter 27 is
reduced to zero, the output of the zero detector 31 is changed to a low
level output.
The output terminal of the zero detector 31 is connected to a circuit 33
for driving the ignition coil 34, and this driving circuit 33 supplies an
ignition current [FIG. 4-(F)] corresponding to an inverting form of the
voltage [FIG. 4-(E)] applied thereto, to a primary winding of the ignition
coil 34, whereby in each of spark plugs 16a through 16d, sparks [FIG.
4-(G)] are generated at every falling point in the above-mentioned
ignition current [FIG. 4-(F)].
As will be apparent from the foregoing description, according to the
present invention, even in an internal combustion engine provided with an
EGR apparatus, since at every operation point of the engine the
spark-advance angle is determined by calculating the EGR ratio, the timing
control can be performed in a manner most preferred for reducing the
contents of harmful components in the exhaust gas and for lowering the
fuel consumption ratio. Especially, since the sum of the amount of intake
air introduced into the engine and the amount of EGR gas is represented by
the actually measured intake manifold absolute pressure value and the
intake manifold absolute pressure value P.sub.BO occurring when the
exhaust gas is not being recirculated, which value P.sub.BO can be
precisely obtained for the entire operational region of the engine by
means of calculation using a particular function, the EGR ratio can be
determined very precisely and assuredly with no substantial error,
therefore, the ignition timing can be very precisely controlled to an
optimum level.
As many widely different embodiments of the present invention may be
constructed without departing from the spirit and scope of the present
invention, it should be understood that the present invention is not
limited to the specific embodiments described in this application, except
as defined in the appended claims.
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